This invention relates to strip media drive systems and more particularly to strip media drive systems having a pair of belt coupled strip media packs for reeling the strip media therebetween. Of particular interest is a belt coupled strip media drive embodied in a magnetic tape cartridge.
The basic principle of operation for magnetic tape drive systems, cartridge or non cartridge, that utilize thin, flexible, endless peripheral belts to transfer the tape from one reel to another under tension, is the two pulley, flat belt drive. In such a belt drive one pulley is the "driver" and the other is the "driven" pulley or the "load".
In such a system there exists a slack (low tension) and a taut (high tension) side of the endless belt, the taut side always being "downstream" of the driven pulley in the direction of motion of the belt and between the driver and driven pulley. Since any elastic material stretches under tension, the taut side of the endless belt is thinner than the slack side and since such belt drives are always endless closed loop systems, continuity of mass flow must exist by the laws of physics. This means that at any point in the belt drive, an equal amount of belt mass must pass per unit time. This then requires that the velocity of the belt on the tauter (thinner) side of a belt drive must be greater than the velocity of the belt on the slacker (thicker) side by an amount proportional to the difference in thickness.
Magnetic tape cartridges have been produced in varying configurations and in a wide range of capabilities for application in audio, analog and digital recording. Early tape cartridge designs utilized capstans with pinch rollers for moving the tape and various configurations of slip clutches or brakes as part of the reel drives to create tape tension. Since application of Coulomb friction in any mechanical system results in high energy losses and poor controllability, the early cartridge left much to be desired in terms of speed accuracy and uniformity for analog recording applications. Also, because of the energy inefficiency, rapid acceleration and deceleration for digital computer applications was not feasible.
Belt driven magnetic tape transports and cartridges are known in which a peripheral endless flexible belt contacts a reel pack (tape wound on rotatably mounted reel hub) in such a manner that motion induced into the belt by external rotating power sources causes a transfer of tape from one reel pack to another. Such devices are described in the IEE transactions on Magnetics, Vol. MAG-14 No. 4, July 1978, pp. 171-175 entitled "An Improved ANSI-Compatible Magnetic Tape Cartridge" by Newell; U.S. Pat. Nos. 2,658,398; 2,743,878; 3,114,512; 3,125,311; 3,154,308; 3,297,268; 3,305,186; 3,467,338; 3,514,049; 3,620,473; 3,692,255 and 3,907,230 and French Pat. Nos. 2,212,981; 2,220,841 and 2,260,165.
In the above mentioned belt driven tape transports and cartridges the belt is preferably separated from the magnetic tape as the tape passes across the magnetic record and reproduce heads to minimize tape motion disturbances due to the differences in elasticity between the belt and the tape. Tension in the magnetic tape is created by causing the belt on the takeup side of the drive to move at a slightly higher velocity than on the supply side as described above. This is accomplished by means of dual differential capstans or braking devices in the belt path.
The peripheral belt driven cartridges described in the prior art do possess the capability of bidirectional tape motion under tension and are completely removable from the tape transport as self contained devices. However, due to the very low modulus of elasticity of the elastomer belts used on some cartridges, which is required to create tape tension, the resonant frequencies of the belt-tape systems are so low as to severely limit the acceleration and deceleration rates or start-stop times of the cartridge tape drive systems. High acceleration and deceleration rates are of vital importance to modern high speed computer tape drives. Rapid acceleration of the tape in prior art cartridges can also result in excessive and harmful tensile stresses in the tape substrate. In addition, the elastic characteristics of the belts do not allow the close control of instantaneous speed variations that is essential for high frequency, precision analog recording and high flux reversal density, high speed digital recording for computer applications.
The prior art generally teaches the construction of peripheral belt driven tape cartridges that require the use of elastomer belts with very low modulii of elasticity and the prior art does not provide for any means for precisely adjusting the effective tape tension after complete cartridge assembly. In the prior art the effective tape tension is invariable after assembly of the device and is dependent on the physical and elastic characteristics of the materials used and the variation in dimensional sizes of the cartridge components due to manufacturing tolerances.
The major means, then, employed in the prior art for creating the difference in tension in the endless belt have been mechanical braking devices, differential dual capstans, Coulomb friction, or the squeezing of a thick rubber belt. A disadvantage of these prior art devices other than the one employing braking means is that no adjustment of the effective tape tension is possible after assembly of the components. The final tension is determined by the manufacturing tolerances of the individual parts and the elastic characteristics and variations of the belt material. The disadvantages of the prior art devices employing mechanical braking means include low reliability and non-uniformity of tension due to uncontrollable and often instantaneous variation of the frictional characteristics of the materials used and the considerable variation of the frictional characteristics with temperature and usage.